Recently, provision of print materials of various kinds and small lots is under progress, and attention is drawn to, as a substitute for off-set printing of a conventional art, inkjet printing which is on-demand printing made easy to respond to high speed printing.
The inkjet printing is a printing method in which ink droplets ejected from a printer head are landed on a recording medium, infiltrated and fixed to form dots, and in accordance with gathering of many of the dots, an image is formed. This process of forming dots is critical for forming a clear image.
Uncoated papers such as high quality paper and common paper generally used as a recording medium has a large amount of hydroxyl groups of cellulose, and thus it is hydrophilic and exhibits a swelling property according to absorption of water. From this point of view, an aqueous ink is generally used as an ink.
Meanwhile, coated papers such as coat paper, art paper, and cast paper are used for a printed materials such as calendar, poster, and color pages of a magazine for which high-level reproducibility is required, and a coating layer is formed by coating a paper substrate surface with a solution (coating solution) which has a binder and a white pigment containing kaolin or calcium carbonate as a main component dispersed in water. As the binder, synthetic latex is mainly used, for example, and a hydrophobic coating layer is formed by filling spaces among pigment particles. For such reasons, when printing is made on coated paper by using a general aqueous ink, the ink does not easily infiltrate the recording medium compared to uncoated paper. Thus, there was a problem that, as the ink is repelled on a surface of coated paper and it cannot be applied homogeneously, the printed material tends to have insufficient water resistance or solvent resistance.
Accordingly, blending a resin emulsion as a fixing agent in an aqueous ink is known, for example. However, even when a resin emulsion is used, according to a known technique of a conventional art, the printed material has insufficient water resistance or solvent resistance.
Furthermore, when a resin emulsion is contained, there is a case in which ejection stability like continuous ejection property or intermittent ejection property is impaired if the emulsion has insufficient stability. In accordance with the enhancement in density and resolution of an inkjet head, and the increase in printing speed, due to development of an inkjet technique in recent years, it has been a subject to ensure ejection reliability.
Conventionally, for synthesis of a resin emulsion, inorganic salts like sodium hydroxide or ammonia is used as a neutralizing agent to ensure the dispersion stability of a resin in a solvent or the solvent affinity of a resin (for example, Patent Literature 1). However, when inorganic salts are used, members of an inkjet printer may be corroded, and ammonia has a problem in terms of storage stability as it has high volatility.
Meanwhile, as an aqueous ink containing amino alcohol, an ink composition which contains a pigment coated with water insoluble resin, an amino alcohol, a water soluble organic solvent, water, and resin fine particles in an amount of 5% by mass or more is disclosed in, for example, Patent Literature 2. For improvement of wiping property during drying and solidifying, an amino alcohol is used in Patent Literature 2. According to Patent Literature 2, it is preferably considered that use of a water insoluble resin having an acid value of 30 mg KOH/g or more is preferred in terms of pigment dispersion property and storage stability.
However, there is still a problem that, even when such an aqueous ink is used for coated paper, a printed material exhibiting sufficient water resistance or solvent resistance cannot be produced. In addition, for use in an inkjet mode, it is required for an ink to have good ejection stability.